public void actionPerformed(ActionEvent event) {
    if (scene.canRemove()) // If shapes can be removed
    {
      scene.removeSelected();

      // if no more objects can be removed, disable this action
      if (!scene.canRemove()) {
        setEnabled(false);
      }
      // check if the opposites can be enabled.
      checkOpposite();
    } else { // disable this action, check if opposites can be enabled
      setEnabled(false);
      checkOpposite();
    }
  }